1. Field of the Invention
The present invention relates to a magnetic position detecting device for detecting a position of a moving body relative to a fixed body. More particularly, the invention relates to a magnetic position detecting device capable of highly accurately detecting a reference position by use of a magnetism-sensitive element, such as a Hall element or a magnetoresistive effect element.
2. Description of the Related Art
The device for detecting a position of the moving body is used in a monitoring system, for example. In the system, a monitor camera picks up an object to be monitored while being rotated in an appropriate direction. An image thus picked up is recorded or displayed on a screen of a monitor device. To know a direction of the monitor camera, a reference position (position of an origin) for the rotation of the monitor camera is determined in advance, and a quantity of angular movement of the camera (rotational angle) is detected with respect to the reference position.
A magnetic position detecting device 60 shown in FIG. 6(a) is known for the position detecting device used in the monitoring system of this type. The magnetic position detecting device 60 includes a magnet section 63 in which an S pole part 61s of a magnet 61 is bonded to a back yoke 62, and a Hall element 64 which is moved along an N pole part 61n of the magnet 61. The Hall element 64 senses a change of flux density when it moves along the N pole part 61n of the magnet 61. A relative displacement between the magnet section 63 and the Hall element 64 is detected based on the flux change sensed.
FIG. 6(b) shows a graph for explaining the principle of the position detection. Positions Pa and Pb at which a magnetic flux detected by the Hall element 64 is zero in density are present on a traveling path of the Hall element 64. A relative position between the moving Hall element 64 and the magnet section 63 can be detected using the position Pa or Pb as a reference.
In the monitoring system, the magnet section 63 is mounted on one of the monitor camera as the moving body and the fixed body for supporting the same, the Hall element 64 is mounted on the other. The position Pa or Pb where a magnetic flux density is zero in density is preset as a reference position, for example, an original position of rotation of a motor. A direction of the monitor camera can be known from a quantity of movement of the moving body (a quantity of rotation when the moving body is a motor) from the original position.
In the magnetic position detecting device 60, as seen from a magnetic flux density curve shown in FIG. 6(b), a density of a magnetic flux that the Hall element 64 senses when it moves along its moving path, gently varies with its lower limit being zero. Accordingly, in a curve representative of a variation of an output signal from the Hall element 64, a rise and a fall of the curve are indistinct, and the positions Pa and Pb at which the flux density is zero greatly vary, resulting in position detection errors.
To overcome the disadvantage, in a magnetic position detecting device disclosed in JP-A-5-26604, as shown in FIG. 7(a), a back yoke 62 is bent to have a shape like U. With such a structure, as shown FIG. 7(b), a magnetic field is inverted in polarity at the boundaries (neutral points between N- and S-poles) between both ends of the back yoke 62 and the magnet 61. In the magnetic position detecting device, variations of the positions Pa and Pb at which the magnetic flux density is zero are reduced. Further, those positions can clearly be detected by using the polarity inversion. Accordingly, the position detection error is greatly reduced.
However, the magnetic position detecting device is also difficult in stably detecting the positions where the magnetic flux density is zero. The reason for this follows. A voltage (voltage difference) corresponding to a magnetic flux density zero varies by variation of offset voltages of the Hall elements 64, change of an offset of an electronic circuit used for the position detecting device, ambient temperature variation and others. To cope with this, it is necessary to deviate a threshold value of detection to the positive direction by a voltage value corresponding to those variations. Otherwise, the positions of the flux density zero which are located far apart from the magnet 61 will erroneously be detected as the correct ones. For this reason, the threshold value of the detection must be deviated. As a result, the detection positions are affected by variations of the power source voltage, magnetic force of the magnet, ambient temperature and others. Accordingly, the position detected is shifted from the reference position for the position detection as set at the manufacturing stage to thereby cause a position detection error. Resultantly, the device operation loses its normality, sometimes.